The present invention relates to testing procedures that can be used to determine the sensitivity of integrated circuits to ionizing doses of radiation in advance of their actual exposure to radiation of the type and intensity to which they will be exposed in their intended application. More particularly, it relates to a scheme for pre-irradiating silicon integrated circuits at low levels and determining their sensitivity to future radiation exposure at much higher levels. Furthermore, the adverse effects of the low level exposure can then be easily removed, leaving almost no permanent damage. Consequently, the procedure of the present invention is a non-destructive method of performing 100% screening of integrated circuits for ionizing dose sensitivity.
Many military and commercial electronic systems may be exposed to nuclear radiation. Military systems would have to withstand such exposure during nuclear warfare, while commercial space systems will experience this exposure if they operate in natural space radiation. This forces designers of such systems to harden them against the adverse effects of radiation exposure.
It has been shown that the elements of electronic systems that are most sensitive to radiation are integrated circuits (ICs). Consequently, there is great interest in methods for determining the radiation hardness of the ICs to be used in such applications, and assuring that the hardness level, once determined, is maintained during system production and deployment.
2.1.1 Integrated Circuit Technology
The most widely used integrated circuit technology in the world today is one known as CMOS (complementary metal-oxide semiconductor) technology. The most widely used products using CMOS are digital devices, such as memory chips, microprocessors, etc.. Therefore, the following description of the present invention will address digital CMOS ICs, even though the general approach can also be used for CMOS integrated circuits other than digital CMOS ICs.
CMOS technology gets its name from the fact that P-channel Metal-Oxide-Silicon (MOS) transistors and N-channel MOS transistors are used in series in various combinations to implement logic functions. Since, in operation, one of these transistors is turned OFF while the other is turned ON, circuits implemented in such an arrangement dissipate little power, except when they are changing state. This makes this technology interesting in many applications where low power dissipation is important.
FIG. 1 shows a cross sectional view of the PMOS and NMOS transistors that are used in CMOS ICs, with the gate oxide (usually fabricated with SiO2) labeled as such, and the field oxide identified as the SiO2 region. FIG. 2 is a circuit schematic showing how such transistors would be used in the simplest CMOS circuit, an inverter. (Output buffers, commonly used on digital CMOS ICs, use this circuit configuration.) The PMOS device is shown as Q2 in this diagram, and the NMOS device is shown as Q1.
A critical parameter of both N-channel and P-channel MOS transistors is the threshold voltage, VT. This is the value of the gate voltage at which significant current begins to flow from the source to the drain. This parameter is the single most important quantity in CMOS integrated circuit technology. It is significantly affected by exposure to ionizing radiation, and therefore is important to the understanding to radiation damage effects in CMOS ICs, and to an appreciation of the object of this patent.
2.1.1.1 Ionizing Radiation Damage in Digital CMOS ICs
Ionizing radiation degrades digital CMOS ICs by (1) causing changes in the threshold voltage (VTN) of N-channel MOS transistors, decreasing it under some circumstances, and increasing it under others, by (2) causing increases in the threshold voltage of P-channel MOS transistors (VTP), and (3) by causing increases in the ON-resistance (RON) of both types of devices. Decreases in VTN can lead to increases in operating current and/or inability to switch. Increases in VTN and VTP can cause reductions in the speed of operation of a circuit and loss of circuit functionality. Increases in device ON-resistance can lead to inability to drive other circuits, either on-chip or off-chip, with resultant loss of functionality.
The part of a CMOS transistor most sensitive to ionizing radiation is the oxide layer, either the gate oxide or the field oxide. When ionizing radiation impinges on an oxide, the energy deposited creates electron/hole pairs. The electrons (conventionally denoted xe2x80x98nxe2x80x99) are more mobile than the holes (denoted xe2x80x98hxe2x80x99), and are swept out of the oxides in times of the order of a picosecond. In that time, however, some fraction of the electrons and holes recombine. This fraction depends on the applied electrical field and the energy and kind of the radiation, and is small for normal operating conditions.
FIG. 3 shows the processes involved in radiation damage to CMOS ICs, starting with the generation of electron/hole pairs by radiation, the transport of holes through localized states in SiO2 bulk, the trapping of some of these holes near the Si/SiO2 interface and the formation of traps at this interface. The first three of these steps are well understood, but the formation of interface traps remains incompletely understood.
Digital CMOS ICs exhibit radiation-induced failure mechanisms attributable to either positive charge liberated by the radiation and trapped in oxide layers, or to negatively charged traps at the silicon dioxide/silicon interface. The former effect is responsible for the decrease in VTN and the increase in VTP cited above, and for an increase in ON-resistance of both types of devices. The latter is responsible for increases in VTN and VTP and also for increases in the ON-resistance of both types of devices.
Process (1) in FIG. 3 illustrates the generation of electron/hole pairs by ionizing radiation. This process, and the initial recombination that accompanies it, determines the yield of radiation generated holes. This process is well understood, and the yield of charge carriers generated by radiation can be calculated once the rate at which the incident particles lose energy is specified.
Process (2) in FIG. 3 illustrates the dispersive, hopping transport of holes from their point of creation to the vicinity of the Si/SiO2 interface. The transport of radiation-generated holes through oxides usually takes place under an electrical bias field, and it takes place via xe2x80x9csmall polaronsxe2x80x9d. xe2x80x9cSmall polaronxe2x80x9d transport is a temperature-activated process at temperatures above about 160xc2x0 K, and is not temperature-activated below 160xc2x0 K. At temperatures below this transition point, holes are xe2x80x98self-trappedxe2x80x99 at their point of creation, while at higher temperatures, they travel toward the silicon (for positive gate bias).
Process (3) in FIG. 3 illustrates the trapping of holes near the Si/SiO2 interface. Mobile holes eventually encounter a distribution of hole traps that starts at the Si/SiO2 interface and extend a few nano-meters into the oxide. Depending on the local density, Nht, and hole capture cross section, "sgr"ht, of these traps, a fraction fT of the holes will be captured. Hole trapping fractions are usually much less than one, so most of the holes drift out of the oxide into the silicon at normal temperatures, and do not contribute to degradation of the electrical properties of devices. Even though fT is usually  less than  less than 1, it can still vary considerably from one device to another. In fact, the magnitude of the hole trapping fraction, fT is a parameter that differentiates xe2x80x9chardxe2x80x9d oxides from xe2x80x9csoftxe2x80x9d oxides, with the former usually having fT values of  less than 1%, while the latter can have fT values of ≈10%. As a result, most holes generated by radiation do not contribute to electrical degradation at room temperature.
Process (4) in FIG. 3 shows the formation of interface states. The mechanism(s) whereby interface traps are created in CMOS devices by ionizing radiation remains controversial despite many years of research. Models to describe the process fall into two main groups. One group contends that some of the trapped positive charges near the Si/SiO2 interface convert to negatively-charged interface traps by various mechanisms. The other group contends that some of the holes created in the oxide by radiation react in the oxide to produce positively charged ions, probably H+. These ions then slowly drift under positive gate bias to the Si/SiO2 interface where they react with Si-H bonds to form molecular H2 and dangling Si bonds, which are the interface traps.
2.1.1.2 Failure Modes in Irradiated CMOS ICs
As a result of the physical changes that ionizing radiation causes in CMOS ICs, a range of failure modes and mechanisms are observed in irradiated devices. These changes are most easily observed in those elements of the IC that are located at inputs or outputs. Radiation-induced changes in input or output elements which prevent the IC from supplying the specified output drive current levels for digital high and/or digital low output levels, or prevent it from achieving the proper output waveform rise and fall times, or prevent it from meeting specified leakage current specifications, etc., are called parametric failures, and are the most commonly seen radiation-induced failures. Parametric testing is performed to detect radiation-induced parametric failures affecting an MOS transistor at the input or output of an IC.
Some IC designs, however, demonstrate greater sensitivity to radiation damage on the part of interior elements than on the part of elements located at inputs or outputs. When this occurs, changes in current drive capability or leakage of interior elements will cause the IC to fail functionally. In such cases, the IC will cease to perform the function for which it was designed, or will begin to perform this function incorrectly (generate digital errors), even though the input/output characteristics are still within specification limits. Functional testing of ICs attempts to detect radiation-induced faults affecting the logic function of the chip (functional failures). In its simplest form, it consists of functional verification, i.e. verifying that the chip xe2x80x9cdoes what it""supposed to doxe2x80x9d. A well defined digital input signal (called an input vector) is applied to the chip and the output signal produced in response to the input signal is compared to the value from a known-good part for the same input data. Functional testing verifies that the device is capable of performing the function for which it was designed, e.g. digital multipliers are functionally tested by verifying that they perform the multiplier function, microprocessors can be functionally tested by performing built-in self test [BIST] routines that are often supplied with these devices, which exercise many of the on-chip functions required of microprocessors. Functional failures occur when the output produced by an irradiated chip differs from that produced by an xe2x80x9cidealxe2x80x9d chip.
To categorize these effects, researchers have developed four classifications of radiation-induced failures. Table 1 shows these categories, the rates at which the dose is delivered, the failure mechanisms they exhibit for these dose rates, and the cause of these failure mechanisms.
2.1.1.3 Radiation Hardened CMOS ICs
Over the last few decades, many ICs have been designed and built to be xe2x80x9cradiation hardenedxe2x80x9d devices. An entire technology has been developed for this purpose, using special design and processing procedures which assure that the resultant product meets the requirements of the system for which it is intended. The special design techniques used include such procedures as making elemental portions of the IC larger than they would otherwise be in order to reduce the probability of radiation-induced interactions between adjacent components. While improving hardness, this can also mean that the number of components fabricated on a chip of a given size is reduced, reducing performance and increasing chip costs. The special processing procedures used include such steps as reducing the temperature at which critical steps are carried out to avoid the adverse effect that high temperature processing has been found to have on the radiation hardness of ICs. This can mean that radiation hardened chips must be fabricated on special production lines, rather than the lines used to fabricate devices intended for use in commercial applications, where radiation hardness is not an issue. This also increases the cost of radiation hardened ICs and makes it more difficult for the designers of radiation hardened systems to take advantage of the many ICs available in the commercial world.
2.1.1.4 Radiation Hardness Assurance
Radiation Hardness Assurance (RHA, or HA) is an aspect of Quality control that addresses the procedures used to control the variability in radiation hardness of ICs. RHA (or HA) attempts to assure that semiconductor devices retain the radiation hardness attributed to them as a result of characterization testing.
Several factors can cause semiconductors to have ionizing dose hardness less than that of supposedly identical devices. To understand these factors, it is necessary to understand the processes used to design and manufacture ICs. CMOS integrated circuits are processed one lot at a time, making IC production a xe2x80x9cbatchxe2x80x9d process. The finished product is identified by the electrical function it performs and by the input and output (I/O) parameters it exhibits. The ability of an IC to tolerate exposure to ionizing radiation has been shown to depend on the details of the process steps used and on the layout of and the interrelationship between the constituent circuit elements.
Radiation hardened ICs are manufactured with tightly controlled (baselined) processes. xe2x80x9cBaseliningxe2x80x9d a semiconductor process means that, once the radiation hardness of an IC is determined, the manufacturer agrees not to change the product or process in any significant way without first evaluating the impact of such a change. Consequently, all devices manufactured on a fabrication line producing xe2x80x9cradiation hardenedxe2x80x9d ICs are found to be similar in radiation performance and thus constitute a statistically homogeneous population. The radiation hardness of individual integrated circuits then can be presumed (as in the case of a xe2x80x9cQualified Manufacturing Line, or xe2x80x9cQMLxe2x80x9d) or determined by standard statistical methods, such as xe2x80x9csample testingxe2x80x9d. Sample testing consists of drawing a sample of devices from a statistically homogeneous population of devices, exposing them to the environment of interest, and inferring the future performance of all members of the population from the known performance of the sample. While QML lines are preferred, since they xe2x80x9cdesign inxe2x80x9d radiation hardness, sample testing is still done on product from some lines to verify continued radiation hardness.
FIG. 4 shows a method currently favored for screening parts from a statistically homogeneous population for ionizing dose hardness, using sample testing. It is intended to provide a conservative estimate of the hardness of CMOS ICs. The main flow of the test consists of two parts. The first part, labeled xe2x80x9cIxe2x80x9d, consists of an irradiation to the total dose specification, D, at a dose rate of Ro, where Ro is between 50 and 300 rad (Si)/sec. Electrical test follows, within two hours of the irradiation. The second part, labeled xe2x80x9cIIxe2x80x9d, consists of a second irradiation to an additional level equal to 50% of the initial dose, D, followed by a 168 hour anneal at 100xc2x0 C. under bias, and a second electrical test.
While using radiation hardened CMOS devices relieves systems designers of concerns about Radiation Hardness Assurance, they (systems designers) are increasingly unwilling to pay the price associated with the special procedures required to build radiation-hardened integrated circuits. Consequently, interest has grown in the development of techniques for using conventional commercial ICs in applications where exposure to nuclear radiation is a concern. Such a switch in strategy poses numerous problems, however. These problems arise because commercial suppliers of ICs are structured to supply commodity parts at a low price. They produce in large quantities, and are reluctant to change their procedures, even in a minor way, for customers who only want to buy what they consider to be a few parts. (The total radiation-hardened semiconductor business is only about 0.5% of the world-wide semiconductor business.) In particular, they will often be unable or unwilling to maintain wafer lot identity. Since ionizing dose hardness is known to vary from wafer to wafer and from lot to lot, loss of wafer lot identity would call into question the assumption that the parts are all from a statistically homogeneous population, and would invalidate Hardness Assurance techniques based on radiation-testing sample devices from a lot. Consequently, new techniques are needed to provide assurance of the ionizing dose hardness of ICs, if commercial ICs are to be used in critical systems.
In this regard, testing has shown that some commercial ICs have sufficient hardness for use in some military and space applications, but others, nominally identical, do not. Unfortunately, the variability in ionizing dose hardness of commercial ICs is significantly greater than that of radiation-hardened ICs. Further, as indicated, conventional statistical sampling techniques are inapplicable, due to the inability to assure that the sample is drawn from a statistically homogeneous population. Thus, commercial IC production lines are not expected to meet QML requirements for radiation hardness. In particular, commercial ICs differ from QML ICs in several ways, some of which are process-related, and some of which are product-related. Manufacturers of radiation hardened QML ICs identify and control those features of the process that are critical to radiation hardness. However, such features often do not affect electrical performance or reliability, so commercial manufacturers do not control them to the degree needed to maintain ionizing dose hardness. Consequently, the radiation hardness of commercial ICs can vary, even when produced by lines using Statistical Process Control techniques that are quite adequate for meeting commercial product requirements.
In addition to process variabilities, product design and/or chip layout changes can affect ionizing dose hardness. One type of change that occurs often in the commercial IC world is what is called a xe2x80x9cdie shrinkxe2x80x9d, i.e. a reduction in the size of a chip by implementing changes in design rules permitting manufacturers to realize more chips per wafer, thereby increasing revenues. Such changes occur on the average of every 18 months in memory and processor standard products, and less frequently for analog products. In these cases, circuit function is unchanged, but parasitic structures important to radiation hardness may change in unknown ways. Because of the impact such process, product, and/or layout changes can have on ionizing dose hardness, and the inability to use baselining, QML and/or sample testing, other approaches to Hardness Assurance (HA) are necessary to control the variability in ionizing dose hardness of commercial ICs. Few alternatives are available. One alternative is 100% screening using irradiation and anneal.
2.1.1.4.1 Irradiate and Anneal as a Hardness Assurance Technique
Irradiate and anneal (IRAN) is a Hardness Assurance technique in which each IC is irradiated to a low level, the electrical effect of such radiation exposure observed, and the behavior of the IC at a high level of radiation predicted from its response at the low level radiation. Early researchers attempted to develop this technique to detect devices that are more sensitive to ionizing radiation than the rest of the population being studied. It was intended that the damage inflicted in the process be removed by annealing. Devices were irradiated to a suitable dose under representative bias conditions at room temperature. Specified parameters were monitored and the more sensitive parts separated from the harder parts by means of a predetermined acceptance limit. The acceptable devices were then supposedly restored to their pre-irradiation condition by annealing them at an elevated temperature, making what would otherwise be a destructive test into a non-destructive test.
Early efforts to use IRAN failed because the damage introduced by the pre-irradiation could not be removed by annealing. In particular, when irradiations are carried out at room temperature, positive charge is trapped in the oxides. Interface traps are also formed. Trapped positive charge and interface traps produce different effects and anneal at different rates and different temperatures. Further, it is more difficult to anneal away the interface traps at reasonable temperatures. Thus, when digital CMOS ICs are pre-irradiated at room temperature and then annealed, oxide trapped charge, Not, and interface trapped charge, Nit do not anneal at equal rates. This prevents successive irradiate- and-anneal cycles from tracking, making the process highly suspect, and preventing the use of this approach as a 100% screen. While the concept is still very interesting, and would be valuable if it could be perfected, pre-irradiating parts at room temperature did not work. Accordingly, there is still an urgent need for a non-destructive, 100% screening test for ionizing dose hardness. The present invention satisfies that need.
The present invention describes a testing procedure called CR-ANNEAL (for Cryogenic Radiation and Anneal) for testing all IC devices of a given type to determine their ionizing radiation hardness in a non-destructive manner. This is accomplished by exposing the ICs to radiation sufficient to cause failure while they are in a state of enhanced sensitivity (due to irradiating while they are cooled to a cryogenic temperature), warming them to normal temperature to remove almost all of the radiation damage inflicted by the cryogenic irradiation, and then calculating the radiation level at which they would suffer similar failure if the irradiation had been done at normal temperature.
FIG. 5 illustrates the principle on which Cryogenic Radiation and Anneal is based. This figure shows measured values of the gate thresh-old voltage shift (used as a measure of ionizing radiation damage in this test) of an MOS device vs. total dose for a device irradiated at 85xc2x0 K to a total dose of 30 Kilorads (Si), then warmed to 300xc2x0 K and re-measured, cooled back to 85xc2x0 K and irradiated again to 80 Kilorads (Si), warmed back to 300xc2x0 K and again re-measured, cooled back to 85xc2x0 K and irradiated again to 130 Kilorads (Si), and again warmed to 300xc2x0 K and remeasured. The data shows that the gate threshold voltage shifts by about 5 volts for every 30 Kilorads (Si) when irradiated at 85xc2x0 K, but that this shift is almost entirely removed when the device is warmed to 300xc2x0 K. (Warming to room temperature permits the xe2x80x9csmall polaronsxe2x80x9d to move to the boundary of the silicon dioxide, and leave the silicon dioxide.) Furthermore, the device can be irradiated again and again, and the same behavior observed. While it is true that a slight residual voltage shift is seen after the second irradiation and warming, and a greater residual voltage shift is seen after the third irradiation and warming, the ability to put the device in a state in which it has an increased sensitivity to ionizing dose by cooling it to cryogenic temperatures, and to remove almost all the damage inflicted on it by merely warming it back to room temperature is clearly demonstrated.
To apply this technique to CMOS ICs, it is first necessary to identify the parameter(s) to be used as a measure of ionizing dose damage. To do this, one or more integrated circuit parameters with the following properties are selected as monitors of the damage that ionizing radiation creates in the DUT.
(1) They can be conveniently measured non-destructively on a CMOS digital IC as a function of accumulated ionizing dose at cryogenic temperatures and at room temperature. (NOTE: It will be assumed that the measurements to be done at cryogenic temperature will be performed at 77xc2x0 K, since this temperature is easily and inexpensively reached using liquid nitrogen as a coolant. It should be noted, however, that since xe2x80x9csmall polaronsxe2x80x9d have no mobility at temperatures below 160xc2x0 K, any temperature below 160xc2x0 K can be used for the cryogenic measurements. (Temperatures above 160xc2x0 K can also be used, but the time that can be allowed to elapse between exposure to radiation and the measurement of the monitor parameter(s) gets progressively shorter as the temperature increases above 160xc2x0 K, until at temperatures near 200xc2x0 K, this time gets so short that it would be impossible to perform the measurement with accuracy. To avoid the need to control this time, measurements below 160xc2x0 K are recommended.) Also, room temperature measurements, referred to below as RT measurements, or 300xc2x0 K measurements, can be done at any temperature over the wide range found in laboratories.)
(2) They change as a function of the amount of the ionizing radiation that the device receives.
(3) They are quantitative measures of the degradation of some aspects of the device performance by the radiation.
The procedure described in this invention varies somewhat, depending upon whether the device of interest falls in category I or category II of Table 1. As indicated in FIGS. 6 and 7 for devices in category I which fail parametrically and functionally, respectively, the monitor parameters are first measured for the device under test (DUT) at RT. (Functional tests are also performed on devices that exhibit functional failure.). The DUT is then cooled to a temperature less than approximately 160xc2x0 K, e.g. 77xc2x0 K, and these measurements repeated to characterize the pre-radiation performance of the DUT at RT and at 77xc2x0 K. The DUT is then given several increments of ionizing dose at 77xc2x0 K, and the monitor parameters are measured (still at 77xc2x0 K) after each increment of dose so that curves of the selected currents versus dose at 77xc2x0 K can be plotted. (Functional tests are also performed on devices that exhibit functional failure.) FIGS. 8 and 9 illustrate the process for devices falling under category II and failing parametrically and functionally, respectively.
Once failure is observed, the value of the monitor parameters that are measured at this value of total dose can be used as indicators of the damage level required to cause failure. By comparing the changes in the monitor currents caused by radiation when the DUT is in its sensitized state, to the changes in their values between those taken in the normal state before radiation, and those taken in the normal state after the radiation and return to the normal state, it is found that the change in these currents in the sensitized state exceeds the change in the normal state by a significant factor. This factor is called the relative effectiveness (R.E.), and states quantitatively how much more effective radiation at cryogenic temperature is in damaging CMOS ICs than is radiation at room temperature. Since the irradiated devices were damaged sufficiently either to fail to meet the limits adopted for the monitor parameters, or to fail functionally, upon receiving a dose DFail, cold at cryogenic temperature, and since the damage at the low temperature is more effective by a factor R.E. than is damage inflicted at room temperature, then one can safely predict that the dose required to produce a similar failure at room temperature would be greater than DFail, cold by a factor R.E. The user then compares the predicted dose-to-failure at room temperature DFail, warm (DFail, warm=DFail, cold times R.E.) to the radiation specification for his application, and decides whether it is sufficiently hard for his application. The devices found by this procedure to be sufficiently radiation hard are used, while the others are rejected.
After completion of the above radiation series at 77xc2x0 K the DUT is warmed to RT, and the monitor currents re-measured (and functional measurements, if necessary). It is expected that the values of the monitor parameters read at RT will be close to the pre-rad values of these parameters measured at RT, i.e., little residual damage to the DUT should remain due to the 77xc2x0 K irradiations.
Prior research on the radiation response of SiO2 has shown that the measured values of the RT monitor parameters, after warming from 77xc2x0 K, are the values that the DUT would exhibit if it had been irradiated at RT to the maximum dose that the device received at 77xc2x0 K. Similarly, the 77xc2x0 K doses that would produce the same currents at 77xc2x0 K as the measured RT currents can be read from the measured curves of monitor parameter values versus dose at 77xc2x0 K. Therefore, for each of the monitor parameters, the relative effectiveness (RE) of the radiation in damaging the DUT at 77xc2x0 K and RT is the ratio of the doses that produced equal currents at 77xc2x0 K and RT.
Thus, for a specified value of any of the monitor parameters, the dose that would produce that value at RT can be estimated as follows. From the measured curve for that parameter versus dose at 77xc2x0 K, read the 77xc2x0 K dose required to produce that parameter value. The RT dose to produce that value is then the 77xc2x0 K dose to produce that current multiplied by the relative effectiveness of radiation in producing this parameter value at 77xc2x0 K and RT.
If the application of interest is one for which the time-dependent buildup of interface traps will not cause device failure (e.g. if the ionizing dose is introduced in a short period of time, and the mission duration is also short, as in the case of applications in a strategic missile), or is one in which the user does not expect a significant amount of rebound in the gate threshold voltage for the NMOS transistors (due to the time-dependent buildup of interface traps after the conclusion of the irradiation), the experimental part of the screening procedure would be completed at this point. However, if interface traps are expected to be important in the intended application, additional steps are necessary. To speed up the growth of the interface traps the DUT would be annealed for 168 hrs (1 week) at 100xc2x0 C. The DUT would then be returned to RT, and the monitor parameters re-measured. The difference between the monitor parameters at RT before and after the 100xc2x0 C. anneal is a measure of the importance of interface traps on the performance of the DUT. If they are found to be important, then DFail, warm must be calculated using data from a monitor parameter for which the effects of interface traps and oxide charge are additive, e.g. data from PMOS devices.